As is well known, a conventional substrate for use in facsimile systems, printer heads and the like, as shown in FIG. 2, was produced in such a manner that a glaze layer 22 is applied over the whole surface of a ceramic substrate 21 made of alumina or the like. A heating resistance element 23 and electrodes 24 are thereafter printed thereon, and a wear-proof protective layer 25 is formed on the surface. In such a conventional structure, there is no or little difference in levels between the heating resistance element and parts other than the heating resistance element and therefore, it is required to apply pressure to the printing paper when using a substrate of this type. It is necessary to make the pressure considerably large which greatly limits the degree of miniaturization of such systems.
To avoid the above disadvantage, there has been developed a partially-glazed substrate that is manufactured as shown in FIG. 1. A glaze layer 12 is formed as a projection on a ceramic substrate 11 only at a part of the surface thereof which will be the location of a heating resistance element 13. The heating resistance element 13 and electrodes 14 are printed on the top and sides of the glaze layer 12 and a protective layer 15 is formed thereon. As a result, there is a difference in the level of the heating resistance element 13 and the other part. However, in the case where the partially-glazed substrate had a glaze layer having a portion 1.0 mm or less in width in the transvers direction, a vertically upward rising 12a of glaze was often produced at a longitudinal end portion of the glaze layer as shown in FIG. 3, severely limiting the use of the substrate.
The present inventors have developed a partially-glazed substrate having a glaze layer with a rated thickness of 40-60 .mu.m and a surface tension of 250 dyne/cm or more. In this manner, the glaze can be well-formed without producing any irregularity such as the rising shown in FIG. 3 even in a glaze layer having a width of 1.0 mm or less in the transverse direction. That invention has been disclosed in Unexamined Japanese Patent Publication No. 59-156979.
In order to obtain a predetermined difference in level between the heating resistance element 13 and each of the other parts of the substrate which is required in a hyperspeed facsimile, i.e., one capable of transmitting an A4 page in 2-3 sec., it is necessary to make the rated thickness of the glaze 60-80 .mu.m. The invention disclosed in Unexamined Japanese Patent Publication No. 59-156979, however, is unsatisfactory for forming a glaze layer having a width of 1.0 mm or less and a thickness of 60-80 .mu.m.